This invention relates to metallurgical alloys and, more particularly, to a composite alloy containing hard tungsten carbide particles which may be processed by melting.
Many materials are subjected to wear or erosion during service in machinery, when they contact an environment containing particulate matter. A few examples from the thousands of wear-inducing situations include drill tools, pump impellers, and scraper blades. As the materials wear, increasing amounts are removed until the dimensions of the part become so reduced that it is no longer operable. The part must then be replaced or repaired, usually necessitating that the machinery be removed from service.
One of the ongoing quests in the metallurgical industry has been to identify and exploit materials that resist wear and erosion. An important class of wear-resistant materials includes metals and compounds that are very hard. These materials are useful because they physically resist the scraping action of the erosive environment.
The hardest materials that are commercially practical for large-scale use are compounds such as tungsten carbide. While such materials are very hard and hence wear resistant, they suffer from the drawback that they are usually also quite brittle. That is, even though the material can resist wear, it may fail prematurely through another mechanism such as thermal cycling that requires a degree of ductility for damage resistance. Also, it is difficult to form such compounds into a useful article shape because of their high melting points and low ductility.
To achieve a compromise between hardness and ductility that permits the formation of useful shapes, a class of composite materials has been developed wherein particles of the wear-resistant compound are embedded in a matrix of a more ductile alloy. These composite materials, sometimes called bonded carbides, thus contain two distinct and identifiable major phases, fine particles of the hard material that have low ductility, and a continuous, somewhat ductile phase that is less wear resistant than the hard particles. The most commonly used member of this class is tungsten carbide/metal composites that have tungsten carbide particles embedded in a cobalt, nickel, or metal alloy matrix.
These bonded carbide materials are formed by powder metallurgical techniques wherein particles of the hard phase are mixed with powders of the matrix phase. The two phases are pressed together at an elevated temperature below the melting point of the matrix phase in a hot pressing operation, or heated above the melting point of the matrix in a liquid phase sintering operation. The powders are thus brought into intimate contact as a single material, wherein the hard phase and the matrix each retains its own physical identity. Using powder mixing techniques, the relative amounts of the two phases can be adjusted over a wide range.
Unless the amount of the carbide phase is sufficiently great that it forms a rigid skeletal structure upon melting of the matrix, the bonded carbide materials cannot be formed by techniques that melt the matrix phase using presently available technology, because the hard particles are normally much denser than the matrix material and will sink in the melt. If the mixture is heated to a temperature above the melting point of the matrix but below the melting point of the hard particles, in those cases where a skeletal structure is not formed the hard particles settle because of their higher density, forming a layered structure that is carbide-enriched at the lowest points and carbide-depleted at the highest points. While a skeletal structure may be desirable in some instances, its formation prevents the free flow of the melted mixture, which is a desirable feature in most casting and welding operations, for example.
Material processing operations that include melting are common, and are one of the most economical and practical ways of forming useful structures. Melting operations include conventional casting, but also other techniques not commonly thought of as casting operations but in which melting occurs, such as welding. Welding is a preferred approach to forming hard facings of many wear-resistant materials to prevent erosion of underlying substrates or to repair damaged substrates, but unfortunately cannot be used for bonded carbide materials due to the necessity for flow and the resulting density segregation phenomenon. In existing approaches, if the fraction of carbide phase is sufficiently high to form a skeletal structure to prevent gravity-driven segregation, the melt will not readily flow. If the fraction of carbide phase is reduced to permit flow, gravity-driven segregation leads to an irregular and nonhomogeneous microstructure.
For many applications there is a need for a bonded carbide material that can be processed by operations that include melting of the matrix phase and free flowing movement of the melted composite. Cast wear-resistant parts and welded hard facings of such materials would then be practical. Such a material would desirably permit a variation of the content of the hard phase over a range of values and would have a matrix that is itself wear resistant to avoid undercutting of the hard particles in the wear-producing environment. The present invention fulfills this need, and further provides related advantages.